1. Field of the Invention
The present invention relates to a vehicle fuel supply system that pressure-transmits a fuel in a fuel tank to an injector and the like of a vehicle internal combustion engine and particularly to discharge of gas such as vapor from a pump flow path.
2. Description of the Related Art
In order to trap foreign substances such as grit and dust included in a fuel and to stably supply the fuel, for example, to the injector of a fuel injection device mounted in an engine, foreign substances in a fuel tank are removed, at first, by a relatively coarse strainer, such as a mesh formed of nylon or the like, that is provided at the inlet of a fuel pump. It is widely known that foreign substances that pass through the strainer or foreign substances including abrasion powder, of a brush or a commutator, that is produced in a motor unit incorporated in the fuel pump, are trapped by a high-pressure filter, such as a paper-made filtration element, disposed at a downstream side of the fuel pump (at the next stage of the fuel pump in terms of the fuel flow path).
For the purpose of performing the foregoing stable supply of a fuel to an injector, it is indispensable to boost the pressure of the fuel in the pump flow path; however, when at this time, vapor-including air bubbles (referred to as vapors, hereinafter), as fuel vapors, that are produced because the fuel pressure is high are discharged toward the injector along with the fuel, an error in the amount of fuel injection by the injector may be caused. Accordingly, it is widely known that a vapor discharge outlet is provided in a pump flow path (for example, refer to Patent Document 1).
Meanwhile, in the case where due to a change with time, the intake pressure loss of a strainer increases, the pressure in the vicinity of the vapor discharge outlet becomes negative and hence the fuel is taken in through the vapor discharge outlet. It goes without saying that the fuel to be taken in has not passed through the strainer, which is in the original flow path; therefore, grit and dust also intrude along with the fuel, thereby causing abrasion of the pump chamber or clogging in the high-pressure filter. Accordingly, it is known that in a vapor discharge outlet, there is provided a check valve that allows discharge of vapors and prevents the fuel from flowing reversely (for example, refer to Patent Document 2).